It has been known that titanium oxide has a photocatalytic effect that it presents oxidoreductive functions to decompose harmful matters and to show antibacterial properties and a superhydrophilic phenomenon when irradiated with ultraviolet rays, and industrial products using the effect have been actively developed. A photocatalytic reaction occurs in the vicinity of the surface of titanium oxide, so that the industrial products use titanium oxide by making it into a thin film in many cases. Accordingly, a titanium oxide sol made of fine particles of titanium oxide has been widely used as a thin-film-forming material.
The titanium oxide exhibits the photocatalytic effect by the energy of light as its name indicates, and accordingly it exhibits the effect only when irradiated with light such as sunlight and a light of an ultraviolet lamp. As for antifouling properties originating in superhydrophilic properties among the photocatalytic effects, outer stains existing on a film surface having a photocatalytic function can be removed by intermittent irradiation with ultraviolet rays even though the film is not continuously irradiated with the ultraviolet rays. Harmful matters are also decomposed slowly by the intermittent irradiation of light, unless the matters naturally increase. However, the titanium oxide needs to be continuously irradiated with ultraviolet rays in order to keep the antibacterial properties and a deodorizing effect, because bacteria and foul odors multiply and spread while the titanium oxide does not present the photocatalyst effect. When the photocatalyst is incorporated in a product that can be continuously irradiated with light such as an air cleaner and a purifying facility, the product can be always irradiated with light. However, building materials used indoors or outdoors or other amenity-related products do not have their own light source, so that the photocatalyst does not function in a dark place in which there is no sunlight or a lamp. This fact is a big problem for a photocatalytic product which is expected to have the antibacterial properties as well as the effects of decomposing harmful materials and preventing stains due to the photocatalyst, and accordingly, the photocatalyst has been strongly desired to have the antibacterial properties and deodorizing effects in a dark place.
On the other hand, in order to make a product exhibit an antibacterial effect in a dark place having no light therein, the simplest and easiest method is to employ the photocatalyst together with an antibacterial agent other than the photocatalyst. There are various types of compounds in the antibacterial agent, but when the antibacterial agent is used together with the photocatalyst, the antibacterial agent made from an inorganic substance must be used, because the photocatalyst decomposes organic matters. An inorganic antibacterial component includes a metal such as silver, copper and zinc, and many industrial products have been developed, in which the inorganic antibacterial component exists on the surface of a substrate to exhibit the antibacterial properties.
Such an effect of an antibacterial metal can also be utilized in the photocatalyst. Antibacterial properties can be exhibited regardless of the presence or absence of irradiation light, for instance, by making the photocatalyst and the antibacterial metal exist on the same coating film. For instance, a product can present the antibacterial properties in dark place due to films which have been formed by the steps of forming a photocatalytic film containing titanium oxide on the surface of the product, further applying an aqueous solution of a compound containing an antibacterial metal such as various salts of silver or copper on the surface of the photocatalytic film, and heat-treating or reduction-treating the applied aqueous solution; and thereby possess the photocatalyst effect and the antibacterial properties due to metal ions. However, such a method needs surplus steps of application and drying, and obviously increases the production cost. From an industrial viewpoint, it is preferable to obtain the coating film by applying and drying one chemical solution containing the photocatalyst and antibacterial metal at one time. For that, it is conceivable to blend silver and copper that are the antibacterial metals into a coating-film-forming-material containing the photocatalyst.
As a coating-film-forming material, a titanium oxide sol is preferably used as described above, so that it is conceivable to mix silver and copper in a state of aqueous nitrate solutions respectively with the coating-film-forming material. When an aqueous silver nitrate solution is added to titanium oxide sol STS-01 by a trade name (produced by Ishihara Sangyo Kaisha, Ltd.) stabilized by nitric acid, the resultant solution is apparently stable for a while after silver nitrate has been added. However, after a while, the silver component is reduced, gives the sol a color of yellow to brown, and finally even precipitates. When the sol causes precipitation, the coating film is not stably formed from the sol, shows a defective appearance after the sol has been applied due to the precipitates, or causes the unevenness of antibacterial effects. Then, it is conceivable to add the silver component to the sol right before producing the coating film for the purpose of preventing coloration and precipitation, but it is not practical. For this reason, the present inventors made an extensive investigation on a method of producing a titanium oxide sol which includes an antibacterial metal, has a photocatalytic function and has superior storage stability.
As for a titanium oxide sol containing copper that is one of antibacterial metals, the present inventors have already disclosed the technology. According to the technology, copper is stably dispersed in the titanium oxide sol in a state of a complex dissolved in alkanolamine. (See Patent Document 1)
However, as for a titanium oxide sol containing silver and having the photocatalytic function, a stable sol containing silver without causing discoloration and the production method therefor have not been reported. This is because silver is more easily reduced than copper.
On the other hand, an antibacterial agent formed of a colloidal solution of an antibacterial inorganic oxide is disclosed as an antibacterial agent made of an inorganic colloid, which contains inorganic oxide colloidal particles with negative electron charge having one or more antibacterial metal components selected from the group consisting of silver, copper, zinc, tin, lead, bismuth, cadmium, chromium and mercury, thereon. (See Patent Document 2)
Patent Document 2 describes TiO2, but does not direct at imparting a photocatalyst function to an article, and presents an example of a support of an antibacterial agent. Patent Document 2 also describes that when titanium, zirconium and zinc components are used in combination, the components work as ultraviolet absorbers and show an effect of preventing the silver component from discoloration. However, when titanium has a photocatalytic function as in the case of the titanium oxide sol, titanium works completely inversely. Specifically, the titanium oxide sol reduces silver much more rapidly than a sol of amorphous titanium oxide having no photocatalytic function due to the strong oxidoreduction power of the photocatalyst, and causes discoloration therein.
For instance, when a sol is prepared by the steps of employing Tynoc A-6 by trade name (containing 6 mass % TiO2, having pH of 11 and produced by Taki Chemical Co., Ltd.) as an anatase type titanium oxide sol with negative electron charge, and adding silver oxide dissolved in ammonia to the sol, the resultant sol is stable, and is not thickened or gelated. However, when being irradiated with a light of a fluorescent lamp level, the sol causes discoloration in an extremely short period of time, and causes the precipitation of reduced black silver after a while. Such discoloration and precipitation phenomenon greatly decreases not only the function of a product but also the commercial value such as an appearance and convenience in use.    [Patent Document 1] Japanese Patent Laid-Open No. 2002-68915    [Patent Document 2] Japanese Patent Laid-Open No. 6-80527
For this reason, the present inventors made an extensive investigation on a stable photocatalytic titanium oxide sol which does not cause discoloration or precipitation due to the titanium oxide sol of a photocatalyst in spite of containing silver as an antibacterial metal, and as a result, have accomplished the present invention which will be described in detail below.